Light amplifying device



sepnzz, 1959 E, KAZAN 2,905,830

LIGHT AMPLIFYING DEVICE Filed Dec. 7. 1955 lL-157. 3 I BY BIENQHMIN KHzHN Patented Sept. 22, 1%59 LIGHT AMPLEYING DEVICE Benjamin Kazan, Princeton, NJ., assignor to Radio Corporation of America, a corporation of Delaware Application December 7, 1955, Serial No. 551,542

i12 Claims, (Cl. Z50-213) This invention relates to light amplifiers and particularly to light amplifiers having improved sensitivity. Also, this invention relates to devices that amplify light and devices that will also store the light in half tone quantities.

This invention contemplates the provision of devices for use in promoting the production and storage of light by a luminescent body under the controlling influence of a variable reactance device, with the reactance being varied by input signals. It is known in the electronic arts that a luminescent body can be made to produce light by the application of an electric iield across the luminescent body. This phenomenon is known as electroluminescence. The theory of electrolurninescence is not well understood. However, it seems to be agreed that electroluminescence results from a redistribution of electrons in the crystal structure of the electroluminescent material and the consequent emission of radiation from such material.

There are devices presently known which may be used to produce light images in response to an energizing force of one kind or another. Light storage devices are also well known in the art. However, the known devices are complicated; some of them have a relatively low sensitivity; others have a comparatively low response speed; while others have a relatively low light output or picture resolution. Some of the storage devices that are known are not capable of storage of half tone shades, i.e. the storage of intermediate shades between black and white.

Accordingly, an object of this invention is to provide an improved light amplifying device.

It is another object of this invention to provide a novel light amplifying device of increased sensitivity, speed of response, or light output.

It is a further object of this invention to provide an improved light amplifying and picture storage device that is capable of reproducing half tone shades.

In general, the purposes and objects of this invention are accomplished by the provision of a novel electroluminescent panel including new and improved elemental units. Each off the novel elemental units comprises a photoconductive element, a ferro-electric element and an electroluminescent element. A variable eld is applied across the electroluminescent element by varying the impedance of the ferro-electric element in response to current through the photoconductive element. The current through the photoconductive element is varied in response to input signals.

The Vinvention will be more clearly understood by reference to the following specification when read in conjunction with the accompanying single sheet of drawings wherein;

Figure 1 is a schematic representation of an elemental unit of an image intensifying device in accordance with this invention;

Figure A,2 .is an enlarged fragmentary perspective view,

partially in section, of an electroluminescent panel in accordance with this invention;

Figure 3 is an enlarged fragmentary sectional view of an embodiment of an electrolurninescent panel in accordance with this invention; and

Fig. 4 is a fragmentary plan view of the structure of Fig. 2.

Referring now to Figure l there is shown a schematic representation of an elemental unit of an electroluminescent image intensifying and/or storage device in accordance with this invention. The elemental unit includes a pair of ferro-electric elements 10 and 12 that are connected in series with a pair of electroluminescent elements 14 and 16. The ferro-electric elements are dielectric elernents that change their reactance in response to the voltage across the elements. The electroluminescent elements are elements that produce light in response to a voltage applied across the electroluminescent element. The series circuit of the ferro-electric elements 1@ and 12 and the electroluminescent elements 14 and are connected across the secondary 13 of a transformer 20. The primary 22 of the transformer 26 is connected to a source 21 of alternating current. The secondary coil 18 of transformer 2li has a grounded center tap 24. Connected between the ferro-electric elements 1d and 12 is one side of a photoconductive element 26. The photoconductive element is an element that has a high impedance in the dark, with the impedance thereof being decreased by light striking the photoconductive element. The change in impedance is largely due to a change in resistance in response to light. The other side of photoconductive element 26 may be connected to one side of a source 28 of direct current, or to ground by means of switch 30. Also connected between the ferro-electric elements 1t) and 12 is one side of a resistor 34. The other side of resistor 34 may be connected to ground or to the positive side of another source 36 of direct current by means of a switch 32. Shunting each of the electroluminescent elements 14 and 16 is a leakage resistor 38 and 40 respectively.

With the photoconductor 26 in the dark, with the switch 30 connected to the positive side of the direct current source 28, and also with the switch 32 connected to ground, the circuit of an elemental unit, as shown in Figure l, operates as follows: The alternating current source applied to the primary of transformer 22 establishes a varying electric field across each ferro-electric element 10 and 12 and its associated electroluminescent element 14 and 16 respectively. The magnitude of the alternating current source is selected so that the ferroelectric elements operate in their low impedance range. The electroluminescent elements are selected to have a high impedance, as compared to the ferro-electric elements, so that most of the alternating current voltage is thus applied across the electroluminescent elements which causes the elements to luminesce. When in the dark, the impedance of the photoconductor 26 is high as compared to the impedance of resistance 34 and therefore the direct current potential at point 42 is substantially zero.

When light is focused on the photoconductive element 26, the resistance of the element 26 is lowered and there is an increased direct current potential applied at point 42, i.e. between the ferro-electric elements 10 and 12. It should be noted that this direct current potential may be either positive or negative with respect to ground. This increased direct current potential causes a direct current charge to flow into both of the ferro-electric elements 10 and 12. Substantially no direct current Voltage is built up across the electroluminescent elements 14 and 16 because of the leakage resistors 38 and 40 respectively. This direct current voltage, superimposed upon the v1v1/ternating current voltage in the series circuit connected across the secondary 18, changes the impedance of the ferro-electric elements and 12 so that the ferro-electric elements now have a high impedance as compared to the electroluminescent elements 14 and 16. Due to the increased reactance of the ferro-electric elements, there is an increase in potential drop across the ferro-electric elements 10 and 12, The increase in potential drop across ferro-electric elements 16 and 12 decreases the potential drop across electroluminescent elements 14 and 16 at least to the point where the electroluminescent elements 14 and 16 produce less light. When the potential drop across the ferro-electric elements 10 and lf2 is high enough, the luminescence from the electroluminescent elements will be cut off completely. This condition, in a total panel, is that of forming a dark image on a light background. When light on the -photoco'nductor 26, which caused vthe flow of charge producing the increase in direct current potential at point 42, is removed, the 'photoconductor Z6 returns to its original high impedance condition, and the charge is trapped for a period of time. Hence, the change in light from the electrolurninescent element is stored.

If switch 30 is connected to ground and switch 32 is connected to the source -of direct'currentpotential 36, the operation will be in reverse. -In other words, when no light is focused on the vphotoconductive element 26, direct current voltage will beappliedlat point 42 and there will be no light produced by theelectroluminescent elements 14 and 16 since the majority of the potential drop across the secondary'of transformer 20 will occur across the ferro-electric elements 10 and 12. In this situation most of the potential drop occurs across the ferro-electric element 1t? or l2 because of thefdirect current potential bias produced by source 36. When light is focused on the photoconductor 26, there is a low resistance for the conduction of the direct current, from source 36, to ground,l and thus the bias voltage is removed. VWith the bias voltage most of the potential drop occurs across the electroluminescent element 14 and 16 and therefore, these elements produce light. Thiscondition,in a complete panel, is that of forming alight image'on a dark background.

Referring now to Figure 2 there is shown an enlarged fragmentary perspective view of Van electroluminescent panel comprising a plurality ofthe elemental units shown in'Figure l. The panel46 comprises a transparent support member or glass plate 48 that supports on one surface a plurality of transparent conductive strips Stl. Covering the transparent conductive'strips 50 is a layer ofV electrolurninescent material 52"which in turn is covered by a layer of ferro-electric material "54. Spaced apart on the layer of ferro-electric material 54 is a plurality of conducting squares V56. Extending from the center portion` of one row of squares56 to the center of 'an adjacent row of conducting squares is a triangular shaped strip of photoconductive material 58. In between `each strip of photoconductive `material V52% and extending onto the conducting squares 56 is a triangular shaped strip of resistive material 60. Each of the vstripsof photo- Vconductive material 58 and each of the strips of resistive 'material 60 has a strip 61 of transparent conductive material for purposes of electricalconnection.

Each of the resistive strips 60 is connected vto ground, while each of the photoconductivestrips 58 is connected to thepositive side of the source-62` of direct current potential. The alternate conductive strips tl-are connected to one side of the secondary of a transformer 20' while the intermediate of the conductive strips 50 are connected to the other side of thesecondary of transformer 'It should be understood that the "thicknesses of the 'layers in'panel '46 are shown. greatly' exaggerated for sim- 'plicity ofillustration whilein'actual' practice the'layers are Irelatively thin. 'Thepnel46'n1aylbe'constructed of' the following materials and processes. Thetransparent support memberl 48 may be of a material such as Pyrex glass and may be approximately one quarter of an inch in thickness. The transparent conductive strips Si?, which may be about 10 mils wide and spaced 5 mils apart, may be of a material such as tin oxide or tin chloride and may be deposited by any known technique through a suitable mask to provide the separate strips. The transparent conductive strips 61 may also be of a material such as tin chloride or tin oxide vdeposited through a suitable mask. The electroluminescent layer 52, which may be approximately 1 mil' in ztliicktleSS, may be any of 'the known electroluminescent phosphors such as copper activated zinc sulphide phosphor and may be deposited by any ofthe known techniques such as settling or silk screening. The ferro-electric layer S4, which may be approximately 5 mils in thickness, may be of a material such as Rochelle salt, barium titanate, barium strontium titanate or the like and may comprise a sintered layer or may be grown as a single flat crystal. The {photoemductive strips 53 may be of a material vsuch .a cadmium sulphide or cadmium selenide andmay 'be 1in .the powdered or solid form. As `is known, a :solid ;la yer of photoconductive materials may be deposited byevaporation; while the powdered form `of aphotoconductive vmaterial may be held in a plastic binder, -such as polystyrene, and machined or .molded into -thetriangular shape strips shown. The resistive strips I60 may rb eiorrned 1in ,ally well known manner and 'may be of a material gsuoh as carbon particles in a binder. The resistivestrlips 60 should have a resistivity-of approximatelythatfof-the Photoconductive strips 58, whenfin the-dark, or-preferably lower. For example, the resistivity .of the -strips 60 may be approximately one tenth of-that of the5photoconductor-in the dark. The conducting squares :56, which may :be about l0 milssquare and'spacedapart ,abouti mils,may be ofiany materialsuch saspgold, orsilver. The-,conducting squares 56 may be deposited thy yevaporating materials through a suitable mask.

No specific provision isrm-adelin panel 4,6 for leakage resistance, similar to resistor :38 ,and A0 of -Figure ,1, round the .electroluminescenttlayer 52. ,The reasonfor this is that the layer 52.wi1l.have;a1certain .amountof leakage resistance. :if this resistance isftoo high, lthe electroluminescent; pho sphor may -'be mixed with a, conducting powder sucnz'as tine.carbonparticlesfor,the like.

Those portions of thef:layers.:52,:and:54 :between-acon- Vductive .strip .53a :and conductive .square 56a, for; example, constitute an electroluminescent element and aferroelectric element connected electrically :in series. 'Stmk larly, those portions .of the layers g52rand-f54 :between adjacentA conductive stizip '59h 4and .the conductive square .56a 4constitute:another velectroluminescent element and another ferrofelectliceelement:connected:electrically in series.

rlhe Aoperation tof .the cpanel 46 -fis r substantially 1 the same as thatof atplurality'tothewelementalaunitsshown in Figure l. Thus-Withmolightonthephotoconductor,

4and as was describedmin .connectionwith'igurel, the

transformer .20' f appliesvan :alternating voltage from -a conducting strip through theelectrolurninescent .layer .'32 through the ferro-electric layer S410 aconducting square4 S6, back .through .the ferro-electric layer.,.54 and theelectroluminescent 1ayer.52- .tocan adjacent conduct ing strip`56. VThis alternating Voltage produces an alternating current from 4a strip`50 "through the'layers'to a conducting square'56 back throughxthe layersto the adjacent transparent conductive stripS. Thusycurrent is conducted transversely, fc5-intheirectiorrindicated by dotted lines 57, through the-panel :46. A`SinceV` the impedance*ofthe'ferro-electliclayer54 is low-as com- "pared to the electroluminescent-layer52,-most-ofthe voltage drop is across the electroluminescent rlayer52 'and this layer producesi'li'ght. *It shouldb eunderstood that the'various layers of materials'inepanel46 are 'shown as being greatly enlarged for purposes offillustratom and in actual fact lthese''layers-are'rrelativelyethin. Due

to this thinness, there is substantially no charge conducted between adjacent transparent conductive strips 50 laterally through the insulating electroluminescent layer S2. If there should be a noticeable amount of charge conducted laterally between adjacent strips 50, the area between the strips 50 may be filled with a nonluminescent insulating material (not shown) such as glass.

When light from a scene, or image, to be reproduced is focused on the panel 46, the photoconductive strips 58 become conductive in the areas that are irradiated by the light. As is known, the resistance of the photoconductive layer 58 decreases in accordance with the amount of light that is focused thereon. When the photoconductor becomes conductive a direct current bias voltage is applied from source 62 to the conducting squares 56 in the elemental areas struck by the light, which produces a current flow through the photoconducting strips 58 to the conducting squares 56. This bias varies the resistance, and thus the voltage drop, across the areas of the ferro-electric layer 54, as described above, that are affected by light from the scene, or image, to be reproduced. The bias therefore, due to the change in the resistance across the ferro-electric layer 54, decreases the light emitted by the electro-luminescent layer 52 in the areas illuminated by light from the image because of the fact that most of the potential drop in the alternating current circuit now occurs in the ferro-electric layer 54. This decrease in light is a darkened picture of the image on a light background, which is viewed from the glass plate side of the panel 46. When the light is removed, the photoconductive strips 58 return to their high impedance condition, thereby causing the picture to be stored.

When desired, the strip of resistive material 60 may be formed of a photoconductive material, and these strips kept in the dark. One means of doing this is to form the resistive strip 60 of a rst photoconductive material while photoconductive strips 58 are formed of a dierent photoconductive material, with the materials being responsive to different wavelengths of light. When this is done the panel 46 may be used with either of the wavelengths of light, and one of `the two sets of photoconductive strips will then operate as a resistive strip.

Those portions of the layers 70 and 72 between a conductive strip 68a and a conductive square 74a, for example, constitute an electro-luminescent element and a ferro-electric element connected electrically in series. Similarly, those portions of the layers 70 and 72 between an adjacent conductive strip `6812 and the conductive square 74b constitute another electroluminescent element and another ferro-electric element connected electrically in series.

In the embodiment shown in Figure 2 it is assumed that there is substantially no light feedback between the electroluminescent layer 52 and the photoconductive strips 58. This may be accomplished by: (l) using an opaque ferro-electric layer 54; (2) using a photoconductor 58 that is not sensitive to light of the wavelength produced by electroluminescent layer `52; or (3) by using an opaque layer (not shown).

When it is desired to reproduce a white picture on a black background the connection to ground of the direct current source 62 is reversed so that one side of the photoconductor 58 is grounded and one side of the resistive strips 60 is connected to the source 62. This method of operation was explained in connection with Figure 1.

Referring now to Figure 3 there is shown an enlarged fragmentary sectional view of an embodiment of this invention comprising an electroluminescent panel 64. The panel 64 comprises a transparent support plate 66 having a plurality of spaced apart transparent conducting strips `68 on one surface thereof. Covering the transparent conductive strips `68, and the areas of support plate 66 therebetween, is a thin layer of electroluminescent material 70. Covering the layer of electroluminescent material 70 is a layer of ferro-electric material '7-2. Spaced apart on the layer of ferro-electric material 72 is a plurality of conducting squares 74. Covering the conducting squares 74, and the areas of ferro-electric layer 72 therebetween, is a layer of photoconductive material 76. Covering the layer of photoconductive material 76 is a transparent conducting layer 78. The materials and method of construction, for panel 64 may be similar to those previously described in connection with the panel shown in Figure 2.

It should be noted that the panel 64 includes a continuous photoconductive layer rather than the strips of photoconductor and resistive material as described in connection with panel 46. This arrangement is useful for the storage of pictures. For example, with the alternating current voltage applied across adjacent strips 68 by source 21', and with the panel 6d in the dark, conducting squares 74 assume an average potential that is substantially equal to ground potential. When no image is incident upon panel 64 light is produced by the electrolurninescent layer 7@ since most of the voltage drop from source 21 is across the electroluminescent layer as was explained above. When an image is directed onto panel 64, and with a direct current voltage applied to the transparent conductor '73, the light produces a low resistance through portions of the photoconductor 76. Because of this low resistance through the photoconductor, the potential of the conducting squares in these areas is varied from its original ground potential. This potential, with respect to ground, on the conducting squares 74, produces a biasing voltage pattern on the ferro-electric element 72. This biasing voltage changes the impedance of the ferro-electric element, as was eX- plained above, and thus the light from these areas will be decreased by an amount depending upon the biasing potential. Since the biasing potential depends upon the decrease of resistance of the photoconductor, the decrease in light corresponds to the input image. The charge pattern is stored once the light is removed from a particular spot on the panel, because the photocondnctor returns to its original high impedance condition and the charge is trapped for a period of time. This picture can be erased by connecting the transparent conductor 78 to ground and flooding the photoconductor with light.

The device of Figure 3 can also be used to amplify moving input pictures by applying a square voltage wave to the transparent electrode 78. It this voltage varies from zero to a positive potential, or from Zero to a negative potential, illumination of the photoconductor with an image causes periodic charging and discharging of the elements in an amount determined by the intensity of the light falling on corresponding photoconductive elements. On the average therefore, the illuminated photoconductive elements will cause an average charge or potential, to exist on the corresponding ferro-electric element; with the level of the charge being proportional to the intensity of the input light. When desired, a stroboscopic light may be iiashed on the panel in synchronization with the Zero portion of the square Wave to insure erasure of the previous image.

The devices in accordance with this invention amplify and/or store light images, in half tone shades, by the combining of the properties of electroluminescent elements, ferro-electric elements and photoconductors.

What is claimed is:

l. An electroluminescent device comprising a pair of electroluminescent elements and ferro-electric means electrically connected to said electroluminescent elements, means forming a pair of series circuits each including one of said electroluminescent elements and a portion of said ferro-electric means, and means connected to said ferro-electric means for varying the impedance of said ferro-electric means.

2. An electroluminescent device as in claim 1 wherein said pair .of electroluminescent elements are formed by elemental portions of a layer of electroluminescent matef rial.

3. An electroluminescent device as in claim 1 wherein said ferro-electric means is formed by a pair of ferroelectric elements each of which is in a different one of said series circuits.

4. An electroluminescent device comprising a pair o electroluminescent elements formed by a single layer of electroluminescent phosphor, ferro-electric means electrically connected to said pair .of electroluminescent elementsand physically formed by a layer of ferro-electric material on said layer of electroluminescent phosphor, means forming a pair of series circuits each including one of said electroluminescent elements and a portion of said ferro-electric layer, and photoconductive means connected to said ferro-electric layer for varying the impedance of said ferro-electric layer.

5. An electroluminescent panel comprising a plurality of spaced apart conductive strips, the alternate ones of said conductive strips being electrically connected together, the intermediate ones of said conductive strips being electrically connected together, a layer of electroluminescent phosphor on said conductive strips, and a layer of variable reactance material responsive to variations in electric field on said phosphor.

6. An electroluminescent panel comprising a plurality of spaced conductive strips, the alternate ones of said conductive strips being electrically connected together, the intermediate ones of said conductive strips being electrically connected together, a layer of electroluminescent 4phosphor on said conductive strips, `and a layer of variable reactance material on said phosphor, a plurality of spaced apart conductive elements on said material, and photoconductive means on said conductors for applying potentials to said elements.

7. An electroluminescent device comprising a support plate, a plurality of conducting strips on said support plate, the alternate ones of said conducting strips being electrically connected together, the intermediate ones of said conducting strips being electrically connected together, a layer of electroluminescent phosphor on said conducting strips, a layer of ferro-electric material on said phosphor, a plurality of spaced apart conducting elements arranged in rows on said material, a plurality of strips of photoconductive material each extending from said conducting elements in one row to Asaid conducting elements in an adjacent row, a plurality of strips of resistive material each extending Vfrom said conducting elements in one row to said conducting elements in an adjacent row and between a pair of said photoconductive strips.

8. An electroluminescent device comprising a support member, a plurality of conducting strips spaced apart on said support member, a layer of electroluminescent material on said conducting strips, a layer of ferro-electric material on said elegtroluminescent material, a plurality of conductors spaced apart on said ferro-electric material, a plurality of strips of photoconductive material each in contact with adjacent ones of said conductors, and a plurality of strips of resistive material each in contact with adjacent ones of said conductors and between said photoconductors.

9. An electroluminescent device comprising a support plate, a plurality of spaced apart Conductive strips on said support plate, a layer of electroluminescent material on said conductive strips, a layer of ferro-,electric material on said electroluminescent material, a plurality of spaced apart conductors on said electroluminescent material, and a layer of photoconductive material .on said conductors.

10. An electroluminescent system comprising a support plate, a plurality 4of conductive strips spaced apart on said support plate, an alternating lcurrent source, the alternate ones of said conductive strips being electrically connected together and to one terminal of said alternating current source, the intermediate ones of said conducting strips being electrically connected together and to the other terminal of said alternating lcurrent source, a layer of ,electroluminescent material O11 said conducting strips, a layer of ferro-electric material ,on said electroluminescent material, a plurality of conductive elements spaced apart on said ferro-electric material, a photoconductor on said conductive elements, and means for applying a direct current potential to said photoconductor.

l1. An electroluminescent device comprising a support plate, a plurality of conductive strips spaced apart on said support plate, the alternate ones ,of said strips being electrically connected together, the intermediate ones of said strips being Velectrically ,connected together, .a layer of electroluminescent material on said strips, alayer of ferroelectric material 011 said electroluminescent material, a plurality ,of conductive elements spaced apart on said ferrofelectric material, a layer ,of photoconductive material on said conductive elements, ,and a transparent conductive coating on said photoconductor.

12. An electroluminescent device as ,inlclaim l, wherein said impedance varying means in a photoconductive element.

` References vCited in the le o f this -patent UNITED STATES PATENTS 2,698,915 Piper Jan. 4, 1955 2,721,808 Roberts et al .Oct. 25, 1955 2,728,021 Blanks Dec. 20, `1955 2,728,815 Kalfaian Dec. 27, V1955 2,743,430 Schultz et al. Apr. 24, v195.6 2,768,310 Kazan et al. Oct. 23, 1956 2,816,236 .Rosen Dec. 10, 1957 am* ...im

, OFFICE CERTIFICATE QF CRRECTION Paremo. 2,905,830 September 2 2, 195? Benjamin Kalzan Column 5, beginning with {'Those portionsn in line 45, strike out all to and including the Words "in ,faerien 'l in line 53; column 6, 4line ll, after l'shovrrn in Figure 2." insert the following paragraph:

Patent should read as orectd below.,

signed and Seaie' this '7th day of June 1960.

(SEAL) Attest:

KARL H. AXLINE ROBERT C. WATSON n Attesting Officer Crmnissioner of Patents 

